PET resin has its greatest use in the packaging industry and especially in the production of bottles for containing a wide variety of carbonated and non-carbonated fluids. One concern in using PET is the barrier properties of the polymer. In the carbonated beverage industry, bottlers are particularly concerned about the rate at which PET bottles allow carbon dioxide, CO.sub.2, to escape from the container. If CO.sub.2 escapes too rapidly, the beverage will go flat on the shelf before it can be sold and used by the consumer. Beverage producers/bottlers, and those who wrap other food products such as cookies, meat, candy and similar products in PET film, are also concerned with the ingress of oxygen from the atmosphere into the product container. While at the present time there are PET resins which can be used in the bottling and wrapping markets, further improvements are desirable.
Among the improvements which have been disclosed in recent years is the incorporation of varying amounts of polyethylene isophthalate into polyethylene terephthalate resins. The resulting PEI-PET resins have been found to have improved barrier properties, thus increasing the shelf life of many products, be it by preventing the escape of contained gases or the ingress of oxygen from the atmosphere.
Polyethylene terephthalate-polyethylene isophthalate (PEI-PET) resin formulations are well known and are described in numerous patents. For example, assigned to Amoco Corporation, Chicago, Ill., is U.S. Pat. Nos. 5,646,208 (a polyethylene terephthalate and polyethylene isophthalate composition); assigned to Mitsui Chemical Corporation is U.S. Pat. No. 5,556,675 [the use of 1,3-bis(2-hydroxyethoxy)benzene {HER} in combination with high levels of PIA]; assigned to Mitsui Petrochemical Industries, Ltd. is U.S. Pat. No. 5,039,780 [a copolyester derived of isophthalic acid, 2,6 napthalenedicarboxylic acid and a dihydroxy compound]; assigned to Mitsui Toatsu Chemicals, Inc. is U.S. Pat. No. 4,417,011 [a resin composition for bonding foundry sand having, among other things, (A) a dicarboxylic acid component of terephthalic acid, isophthalic acid or an alkyl isophthalate, (B) fumaric acid and (C) a glycol component]; assigned to Amoco Corporation is U.S. Pat. No. 5,028,462 [molded plastic bottles and method of producing same using a polyamide composition comprising, among other substances, isophthalic acid and terephthalic acid]; and assigned to Amoco Corporation is U.S. Pat. No. 5,646,208 [a melt composition comprising polyethylene terephthalate and polyethylene isophthalate]. The teachings of all cited patents cited herein are incorporated herein by reference.
While the use of PEI-PET resins is known and becoming increasingly more accepted, there are some inherent shortcomings when PET resins are modified with PIA, particularly with about 8% to about 20% PIA. For example, PEI-PET resins containing about 10% PIA have a very slow rate of thermal crystallization, which is a disadvantage in processing the resin through solid state polymerization (SSP). The slow crystallization rate requires a slower SSP production rate, thus decreasing overall plant output and increasing resin costs. While in some commercial instances the production rate of a 10% PEI-PET article is only 70% of that of PET (only) or 2.5% PEI-PET article, in other commercial instances the SSP crystallization equipment is incapable of processing a 10% PEI-PET resin at any production rate.
Another disadvantage of the 10% PEI-PET resins is that the slow rate of thermal crystallization arising from the use of PIA results in less crystallinity in the less stressed regions of articles such as bottles which may be formed using the resin. This lower crystallinity is particularly prevalent in the neck and base of the bottle, and affects the barrier properties of the bottle. Low crystallinity means that the resulting bottle has reduced barrier properties in the less stressed areas and that contained gases, for example, CO.sub.2, will be able to more readily escape from such areas relative to other areas of the same bottle. Consequently, the full benefit of incorporating PIA into a PET resin is not realized.
Yet another disadvantage of 10% PEI-PET resin is that bottle preforms which use such resin have a significantly higher natural stretch ratio relative to conventional PET (only) and 2.5% PEI-PET packaging/bottle resins. The increased stretch ratio necessitates the use of special bottle preform molds which are shorter and narrower relative to conventional molds in order to accomplish the required stretching. Bottle preforms must be stretched to their natural stretch ratio in order to achieve the strain hardening required to give the bottles acceptable physical characteristics (burst pressure, creep strength and top load strength) as well as barrier performance. The necessity of using special preform molds for PEI-PET resins thus requires expensive retooling of the preform molds and does not permit the use of both PET resin and PEI-PET resin on the same line with the same molds.
Accordingly, it is the object of the present invention to provide novel PEI-PET formulations which overcome the disadvantages presently associated with PEI-PET resins.
It is a further object of the invention to provide PEI-PET resins containing a nucleating agent which accelerates the crystallization process to thereby facilitate faster SSP rates and reduce production costs.
Yet another object of the invention is to provide a PEI-PET resins containing a nucleating agent and a chain-branching agent, the latter being added to reduce the natural stretch ratio of PEI-PET resins to about the stretch ratio levels of commercially available PET resins.